HIV is the causative agent of AIDS (Acquired Immunodeficiency Syndrome). In 2005 the estimated number of individuals living with AIDS worldwide was about 40.3 million. While the rate of HIV infection has started to decrease in several countries, the total number of infected individuals has continued to increase. There were 4.9 million new infections in 2005 and 3.1 million deaths from AIDS-associated illnesses. The therapeutic method of choice to reduce viral loads is Highly Active AntiRetroviral Therapy (HAART). HAART is a therapeutic regime that combines drugs that directly target at least two viral proteins. However, one major problem associated with HAART is the appearance and selection of novel HIV resistant strains. These drugs directly target viral proteins, and thus, become inefficient when HIV mutates. Therefore, there is an urgent need for development of drugs with novel mechanisms of action. In particular, "indirect targets" may represent a solution to this problem. "Indirect targets" are cellular host proteins, whose function is essential for HIV replication. One such "indirect target" is the cyclin T1/CDK9 complex, which is essential for HIV Tat-mediated transactivation. Preliminary studies by this and other laboratories show that CDK9 inhibition in a variety of cells dramatically inhibits HIV replication without affecting cell host proliferation and viability. Based on our preliminary data we hypothesized that highly selective inhibitors of the cyclin T1/CDK9 complex may be effective drugs for HIV therapy with little toxicity. We have partially screened an Onconova Therapeutics' proprietary library of small molecules enriched with compounds exhibiting selective inhibitory kinase activities, for inhibition of purified recombinant cyclin T1/CDK9 activity in vitro. This screening led to the identification of various compounds that inhibit cyclin T1/CDK9 activity in vitro with IC50s in the low micromolar range. Thus, we propose: (1) To complete an ongoing screening of the Onconova Therapeutics' proprietary library of small molecules for inhibitors of cyclin T1/CDK9 activity. (2) To characterize and expand the chemical structure activity relationship of the subset of leading cyclin T1/CDK9 inhibitors identified during the initial screening of the above-mentioned library as well as other compounds, which predictably will be identified. (3) To perform an unbiased high throughput-amenable cell-based HIV transcription screening of this library with the goal of identifying inhibitors of HIV-transcription with no cytotoxicity. Project Narrative: HIV is the causative agent of the Acquired Immunodeficiency Syndrome. The therapeutic method of choice to reduce viral loads is Highly Active AntiRetroviral Therapy (HAART). However, one major problem associated with HAART is the appearance and selection of novel HIV resistant strains. Thus, there is an urgent need for development of drugs with novel mechanisms of action. Our data suggest that the host cyclin T1/CDK9 kinase is a valid target for drug development. We have teamed with Onconova Therapeutics, a small business specialized in development of small molecules for therapeutic intervention, to identify and characterize pharmacological inhibitors of HIV replication with a different mode of action. [unreadable] [unreadable] [unreadable]